User-defined Identifier (UDID) in OpenVMS

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General Information

When OpenVMS is used with Fibre Channel disks, it requires a user-defined identifier (UDID) for each Fibre Channel LUN to detect and name the storage device.

 A UDID is a non-negative integer that is defined on the storage sub-system and used when an OpenVMS device name is created.

This article describes the use of the user-defined identifier (UDID) in an OpenVMS environment. Charon products do not specify requirements regarding this value. The requirements are set by the OpenVMS operating system.

Valid UDID values

Valid UDID values are decimal numbers from 0 to 32766.

If the Fibre Channel disk device is served via MSCP, the device identifier is limited to 9999.

In addition to using a valid number, note that the UDID

  • must be configured on the storage sub-system, and
  • must be unique for the cluster.

The UDID may be configured to match the device LUN, but this is not a requirement.

Disk device names on OpenVMS

A Fibre Channel storage disk device name is created by the operating system from the constant value $1$DGA and the user-defined device identifier, nnnnn. For example: "$1$DGA512".

Fibre Channel disk device names always use an allocation class value of 1 (indicated by $1$). Fibre Channel tape device names use an allocation class value of 2 (indicated by $2$).

OpenVMS treats LUN 0 differently than other LUNs: for allocation class 1 (disk devices), LUN 0 is treated as the Command Console LUN (CCL). It is not displayed as a storage device but as $1$GGAnnnnn device.

Device UDID discovery

OpenVMS detects the UDID value of a device in one of two ways:

  1. Historically, DEC reserved the opcode 0xEC to communicate the UDID from a HSG80 to OpenVMS.
  2. Later, the concept of the UDID was included in the SCSI-3 specification as a variation of IDENTIFYING INFORMATION (A3/05 opcode/subcode). Here, the UDID 0 is not available (i.e., no disk device named $1$DGA0).

Storage sub-system support for opcode 0xEC is only necessary if Fibre Channel is attached via PCI pass-through and the device $1$DGA0 is required by the OpenVMS guest system.

If Fibre Channel is virtualized, Charon provides support for the 0xEC command.

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